Polarized elastic scatter detection method and system of tracking and measuring the velocity of individual aerosol particles

Abstract

Measuring and tracking velocity of individual aerosol particles in a bio-threat detection system are accomplished using a single beam laser source in combination with a birefringent crystal that splits the laser beam into two beams having orthogonal polarization. Scattered light is collected with an elliptical reflector and directed into two detection channels, sampling total elastic scatter in the first channel and sampling polarized elastic scatter in the second channel. The difference in intensity of the scattered light in the polarized channel is used to identify the position of the particles. By taking the ratio of signal output from the polarized detector to the total scatter detector, a threshold level can be established to determine the presence of particles traversing the two beams. The particles are time stamped as they traverse the two beams and the time difference between the pulses can be used to measure the velocity of the particles.

1. A method of bio-threat detection, using fluorescence interrogation applications, to classify particles traversing top and bottom laser beams in a depolarization velocimetric module having a diode laser source and computer automated instrumentation, the method comprising:
 concentrating aerosol particles in a front end aerosol particle concentrator subsystem; 
 interrogating concentrated aerosol particles, using fluorescence interrogator applications, to classify aerosol particles traversing a top laser beam and a bottom laser beam in the depolarization velocimetric module, wherein interrogating aerosol particles includes:
 transmitting a single beam from the diode laser source, 
 splitting, using a birefringent crystal, the single beam into a plurality of beams each of which having orthogonal polarizations, 
 directing collected light of the plurality of beams into a first detector channel and a second detector channel, 
 sampling total elastic scatter in the first detector channel, 
 sampling polarized elastic scatter in the second detector channel, 
 identifying, using differences in polarizations of scattered light, when aerosol particles are traversing one of the top laser beam and the bottom laser beam, 
 determining a ratio of polarized elastic scatter to total elastic scatter, 
 performing data analysis, instrument timing and system control operations in a data analysis, instrument timing and system control subsystem, using the computer processor for outputting trigger signals to a bio-threat particle collector subsystem, and 
 collecting bio-threat aerosol particles in the bio-threat particle collector subsystem. 
 
 
     
     
       2. The method of  claim 1 , wherein interrogating aerosol particles further includes determining separation between beams in the plurality of beams. 
     
     
       3. The method of  claim 2 , wherein determining separation between beams is dependent on characteristics of the birefringent crystal. 
     
     
       4. The method of  claim 1 , wherein interrogating aerosol particles further includes aligning a group of at least two UV interrogator beams. 
     
     
       5. The method of  claim 1 , wherein interrogating aerosol particles further includes exciting fluorescence with the group of at least two UV interrogator beams. 
     
     
       6. The method of  claim 1 , wherein interrogating aerosol particles further includes collecting scattered light with an elliptical reflector. 
     
     
       7. The method of  claim 1 , further comprising outputting signals from a first photomultiplier tube and a second photomultiplier tube to a computer processor. 
     
     
       8. The method of  claim 1 , wherein identifying, when aerosol particles are traversing one of the top laser beam and the bottom laser beam, further includes establishing a threshold to determine if a particle is present in one of the top laser beam and the bottom laser beam and time stamping and assigning velocity to each particle traversing one of the top laser beam and the bottom laser beam. 
     
     
       9. A system for bio-threat detection, the system comprising:
 a front end aerosol particle concentrator subsystem; 
 a bio-threat aerosol particle collector subsystem; 
 a data analysis, instrument timing, and system control subsystem having a computer processor; and 
 an aerosol particle interrogator module including a depolarization velocimetric system, wherein the depolarization velocimetric system includes:
 a diode laser, emitting a single beam for interrogating aerosol particles, communicatively coupled to: 
 a first lens focusing the single beam emitted from the diode laser, 
 a one-half waveplate communicatively coupled to: 
 a first mirror communicatively coupled to: 
 a cylindrical lens focused to form a sheet beam at a focal volume of an elliptical collection optic, 
 a second mirror, communicatively coupled between the cylindrical lens and, 
 a birefringent crystal, which splits the single beam into a group of two orthogonally polarized beams, having a defined separation, and are communicatively coupled to: 
 an elliptical collection optic reflector residing in an interrogator chamber, wherein the elliptical collection optic reflector collects elastic scattered light from aerosol particles interrogated by a top beam and a bottom beam of the group of two orthogonally polarized beams, 
 a second lens focusing collected elastic scattered light into detector channels, 
 a beam splitter, which splits collected elastic scattered light into a first detector channel and a second detector channel, wherein the first and second detector channels include: 
 a first photomultiplier tube, and 
 a polarizer that transmits S-polarized light and a second photomultiplier tube, wherein, the first and second photomultiplier tubes output signals to the computer processor, residing in the data analysis, instrument timing, and system control subsystem, and wherein the computer processor executes a plurality of depolarization velocimetric operations and sub-operations. 
 
 
     
     
       10. The system of  claim 9 , wherein the sheet beam formed at the focal volume of the elliptical collection optic has dimensions of about 70 μm by 3 mm. 
     
     
       11. The system of  claim 9 , wherein the defined separation of the group of two orthogonally polarized beams, is defined by characteristics of the birefrigent crystal. 
     
     
       12. The system of  claim 11 , wherein the defined separation is about 350 μm and wherein the group of two orthogonally polarized beams includes the top beam which is S-polarized and the bottom beam which is P-polarized. 
     
     
       13. The system of  claim 9 , having the computer processor executing the plurality of depolarization velocimetric operations and sub-operations, includes execution of operations and sub-operations comprising:
 splitting the single beam into the group of two orthogonally polarized beams; 
 determining separation between beams in the group of two orthogonally polarized beams; 
 aligning a group of at least two UV interrogation beams; 
 exciting fluorescence with the group of two UV interrogation beams; 
 collecting scattered light with an elliptical reflector; 
 directing collected light into the first detector channel and the second detector channel; 
 sampling total elastic scatter in the first detector channel; 
 sampling polarized elastic scatter in the second detector channel; 
 identifying, using differences in polarizations of scattered light, when aerosol particles are traversing one of the top beam of the group of two orthogonally polarized beams for UV interrogation and the bottom beam of the group of two orthogonally polarized beams for UV interrogation by:
 outputting signals from the first photomultiplier tube and the second photomultiplier tube to the computer processor; 
 determining a ratio of polarized elastic scatter to total elastic scatter; 
 establishing a threshold to determine if a particle is present in one of the top beam of the group of two orthogonally polarized beams and the bottom beam of the group of two orthogonally polarized beams; and 
 time stamping and assigning velocity to each particle traversing one of the top beam of the group of two orthogonally polarized beams and the bottom beam of the group of two orthogonally polarized beams; 
 
 performing data analysis, instrument timing and system control operations in the data analysis, instrument timing and system control subsystem, using the computer processor; 
 outputting trigger signals from the computer processor to the bio-threat aerosol particle collector subsystem, and 
 collecting bio-threat aerosol particles in the bio-threat aerosol particle collector subsystem.